The present invention relates to apparatus and processes for the manufacture of, more substrates specifically to those used for stripping and cleaning silicon wafers.
The importance of clean substrate surfaces in the fabrication of semiconductor microelectronic devices has been recognized for a considerable period of time. Over time, as VLSI and ULSI silicon circuit technology has developed, the cleaning processes have gradually become particularly critical step in the fabrication process. It has been estimated that over 50% of the yield losses sustained in the fabrication process are a direct result of workpiece contaminant Trace impurities, such as sodium ions, metals, and particles, are especially detrimental if present on semiconductor surfaces during high-temperature processing because they may spread and diffuse into the semiconductor workpiece and thereby alter the electrical characteristics of the devices formed in the workpiece. Similar requirements are placed on other such items in the electronics industry, such as in the manufacture of flat panel displays, hard disk media, CD glass, and other such workpieces.
Cleaning of a semiconductor workpiece, and other electronic workpieces, occurs at many intermediate stages of the fabrication process. Cleaning of the workpiece is often critical after, for example, photoresist stripping and/or ashing. This is particularly true where the stripping and/or ashing process immediately proceeds a thermal process. Complete removal of the ashed photoresist or the photoresist/stripper is necessary to insure the integrity of subsequent processes.
The actual stripping of photoresist from the workpiece is yet another fabrication process that is important to integrated circuit yield, and the yield of other workpiece types. It is during the stripping process that a substantial majority of the photoresist is removed or other wise disengaged from the surface of the semiconductor workpiece. If the stripping agent is not completely effective, photoresist may remain bonded to the surface. Such bonded photoresist may be extremely difficult to remove during a subsequent cleaning operation and thereby impact the ability to further process the workpiece.
Various techniques are used for stripping photoresist from a semiconductor workpiece. Mixtures of sulfuric acid and hydrogen peroxide at elevated temperatures are commonly used. However, such mixtures are unsuitable for stripping photoresist from wafers on which metals, such as aluminum or copper, have been deposited. This is due to the fact that such solutions will attack the metal as well as the photoresist. Solvent chemistries are often used after metal layers have been deposited. In either case, limited bath life, expensive chemistries, and high waste disposal costs have made alternative strip chemistries attractive.
Plasma stripping systems provide such an alternative and have been used for stripping both pre- and post metal photoresist layers. This stripping technique, however, does not provide an ideal solution due to the high molecular temperatures generated at the semiconductor workpiece surface. Additionally, since photoresist is not purely a hydrocarbon (i.e., it generally contains elements other than hydrogen and carbon), residual compounds may be left behind after the plasma strip. Such residual compounds must then the removed in a subsequent wet clean.
Ozone has been in various applications in the semiconductor industry for a number of years. Often the ozone is dissolved in deionized water to form an effective treatment solution. The attractive features of such a solution include low-cost, repeatable processing, minimal attack on underlying device layers, and the elimination of waste streams that must be treated before disposal. The main drawback with using such solutions has been the slow reaction rates that translate into long process times and flow throughput.
Photoresist strip using ozone dissolved in water has been somewhat more successful in achieving viable process rate at acceptable process temperatures. However, ozone, like all gases, has a limited solubility in aqueous solutions. At temperatures near ambient, ozone saturation occurs at around 20 ppm. Ozone solubility in water increases dramatically with decreasing temperature, to a maximum of a little over 100 ppm at temperatures approaching 0 degrees Celsius and drops to almost zero at temperatures approaching 60 degrees Celsius. While increasing ozone concentration increases the kinetic reaction rate, a decrease in temperature simultaneously suppresses that rate.
A technique for stripping photoresist and/or cleaning a semiconductor workpiece using ozone and deionized water is set forth in U.S. Pat. No. 5,464,480, titled xe2x80x9cProcess and Apparatus for the Treatment of Semiconductor Wafers in a Fluidxe2x80x9d, issued Nov. 7, 1995. The ""480 patent purports to set forth a method and apparatus in which low temperature deionized water is ozonated by bubbling ozone through the low-temperature water. The low-temperature, ozonated, deionized water is in the form of a bath. Semiconductor wafers are batch processed by immersing the wafers in the bath, for example, to clean the wafers, strip photoresist, etc. However, this method of stripping wafers using ozone has slow process rates due to the low temperature of the deionized water.
Because of the desirability of using ozone in stripping applications, additional prior art methods of using ozonated wafer to strip wafers have been developed. One method includes placing the wafer in an ozone rich atmosphere, heating the surface of the wafer, and spraying cold deionized water on the heated wafer surface. However, as with previous methods of using ozone to strip photoresist from wafer surfaces, this method still has an undesirably slow process rate. This is because not enough ozone is reaching the surface of the wafer to react with and remove the photoresist. This occurs as a result of the deionized water not having sufficient levels of ozone dissolved therein.
Another method that has been developed to strip photoresist from wafers using ozone is to combine the above spraying method with the concept of mist generation. In performing this method, the wafers are placed in a process tank. Ozonated deionized water is then supplied to the process tank so that it fills a bottom portion of the tank. As such, the wafers are not immersed in the ozonated deionized water at all but suspended above the liquid surface. A heater is connected to the process tank so as to be capable of transferring heat to the ozonated deionized water. As the heater provides sufficient heat to the ozonated deionized water, a mist of ozonated deionized water is formed within the process tank. Because the mist consists of very tiny droplets of ozonated deionized water, and because the volume of the process tank above the liquid is filled with an ozone rich atmosphere, the tiny ozonated deionized water droplets absorb more ozone per volume before they contact the wafer surface. As such, when these droplets contact the wafer surfaces, they more efficiently strip the photoresist, resulting in faster processing rates. However, these stripping process rates are still less than optimal. Additionally, apparatus used to perform this method can be expensive due to the costs associated with buying and installing a proper heating element. Still another problem with this method and process tank is that the wafers are positioned in the tank and subjected to the stripping process while in a horizontal position. This impedes the removal of photoresist and increases process time.
Finally, many of these prior art tanks and methods of using ozonated wafer to strip photoresist cannot be used to perform additional wafer processing steps such as cleaning, rinsing, and drying. As such, additional process tanks must be purchased.
Thus, a need exists for a method and apparatus of stripping photoresist from semiconductor wafers using ozone that result in faster process rates and cheaper equipment. Additionally, a need exists for an apparatus that can perform photoresist stripping and other necessary processing steps, such as rinsing and/or drying, in the same process tank.
These problems and others are solved by the present invention which in one aspect is a method for stripping photoresist from integrated circuits comprising: placing at least one wafer having an edge in a process tank having a lid; closing the lid; filling the process tank with a process liquid to a predetermined level below the edge of the wafer; and applying acoustical energy to the process liquid so as to form a mist of process liquid in the process tank.
Preferably, the method further comprises applying acoustical energy to the wafer. It is preferable that the acoustical energy applied to the wafer be the same acoustical energy that is applied to the process liquid. In this embodiment, the acoustical energy applied to the process liquid will pass through the process liquid and across the wafer. This acoustical energy can be created by a megasonic transducer positioned at the bottom of the process tank.
In performing the method of the present invention, it is also preferable that the wafers be placed into the process tank and supported therein in a substantially upright position.
It is preferable for the stripping method to further comprise the steps of spraying the wafer with the process liquid and filling the remaining volume of the process tank with a process gas. It is preferable that the process tank be pressurized when the process gas fills the remaining volume of the process tank. Moreover, it is also preferable that the process liquid be a multi-fluid mixture comprising a liquid and a dissolved gas, the dissolved gas being the same as the process gas. Preferably, the process liquid is ozonated deionized water and the process gas is ozone.
The method of this invention can also comprise steps for rinsing and drying wafers following the stripping steps mentioned above. In the rinsing process, it is preferable that the method described above for stripping be immediately followed by the steps of: resuming the supply of the process liquid to the process tank so as to submerge the wafers, fill the entire process tank and overflow the process tank. After a predetermined time, it is preferable that this rinsing process further comprise: discontinuing the supply of process liquid; and discontinuing the application of acoustical energy to the process liquid and the wafers. Alternatively, this rinsing step can involve injecting chemicals into the process tank as the process tank is being entirely filled and overflowed with the process liquid. In order to dry the wafers, it is preferable that the rinsing step be followed by a drying method comprising: draining the process tank; and blowing hot drying gas on the wafers for a predetermined period of time.
In another aspect, the invention is a process tank having a processing chamber comprising: means to support at least one wafer in the processing chamber; means for filling the chamber with a process liquid; a lid adapted to close the chamber; a liquid level sensor adapted to stop the supply of process liquid to the chamber when the process liquid fills the chamber to a predetermined level below a wafer supported in the processing chamber; and an acoustical energy source adapted to supply acoustical energy to process liquid located in the chamber so as to create a mist of the process liquid in the processing chamber.
Preferably, the acoustical energy source is positioned in the chamber so that upon supplying acoustical energy to the process liquid, the acoustical energy passes through the process liquid and across the wafer""s surface. In this embodiment, the source of acoustical energy can be a megasonic transducer located at the bottom of the processing chamber.
Also preferably, the process tank comprises means to spray the surface of the wafer with the process liquid when the wafer is supported in the processing chamber. It is additionally preferable that the wafer support means support the wafers in a substantially upright position.
It is further preferable for the process tank to comprise means to supply a process gas to the processing chamber, the process gas supply means being located above the predetermined level of the process liquid. In this embodiment, the process gas supply means can comprise a concentration sensor and a pressure regulator. It is preferable that the process gas be under pressure when in the chamber. As such, means to depressurize the chamber can be incorporated into the chamber. In this embodiment, the means to depressurize the chamber can be a properly controlled pressure regulator.
Preferably, the process tank comprises a re-circulation weir, wherein the re-circulation weir is adapted to re-circulate overflowed process liquid back into the processing chamber. Also preferably, the process tank comprises means to drain the processing chamber.
It is also preferable for the means for filling the chamber with process liquid to comprise a mixing device and a concentration sensor. Moreover, the process tank can be adapted to perform a variety of semiconductor processing steps including stripping, cleaning, drying, chemical etching, and rinsing. Finally, the process tank can comprise a temperature sensor.